Generally, zoom lenses are used in television cameras for broadcasting. The general construction of these zoom lenses, in order from the object side, is as follows: a zoom section which has a focusing function and a zoom function, a master section which has an image-formation function, and a stop (i.e., diaphragm). The zoom section has a focus lens group, a zoom lens group and a correction lens group. The master section may also be referred to as a relay section (i.e., a relay lens). A replaceable extender lens may be attached to the master section for the purpose of switching image magnification. The portion of the optical path where the extender lens is attached is normally where collimated light exits, as occurs with an afocal optical system. This is done for the purpose of diminishing the change of optical performance when one switches to a different image magnification by attaching or removing an extender lens.
A zoom lens for a television camera is usually provided with a macro-imaging function and a tracking adjustment function. The macro-imaging function is for the purpose of providing a coarse adjustment of the position of the image plane of the zoom lens, and the tracking adjustment function is for the purpose of providing a fine adjustment in order to precisely locate the image at the receiving surface of the image detector. Traditionally, a method has been known in which a part of the master section is moved along the optical axis for accomplishing both the macro-imaging function and the tracking adjustment function.
More specifically, one known technique is to divide the master section into a front lens group and a rear lens group. The entire rear lens group is moved along the optical axis during macro-imaging and tracking adjustment. Furthermore, in the case that the master section is divided into a front lens group and a rear lens group, the front lens group is normally equivalent to the above-mentioned extender lens and forms an afocal optical system.
A second known technique is to subdivide the rear lens group into, in order from the object side, a first lens subgroup and a second lens subgroup. The first lens subgroup is moved along the optical axis during macro-imaging adjustment, and the second lens group is moved along the optical axis (luring tracking adjustment.
As a third known technique, the master section is divided into a front lens group and a rear lens group, and the rear lens group is subdivided into first, second and third lens subgroups, in order from the object side. The first lens subgroup is moved along the optical axis during macro-imaging (i.e., coarse focus adjustment), and the third lens group is moved along the optical axis during tracking adjustment (i.e., fine focus adjustment).
Recently, in zoom lenses for television cameras for broadcasting, a function has been added to stabilize the image field. Hereinafter, this will be termed vibration control. This is accomplished by dynamically moving a lens element in a direction normal to the optical axis so as to prevent unwanted high frequency rotational movements of the optical axis. It is desirable that the lens element that is dynamically moved normal to the optical axis be located in a section of the zoom lens where the light is collimated so as to not deteriorate the quality of the image. In other words, in the case that the master section is divided into a front lens group and a rear lens group, with the front lens group forming collimated light, it is desirable that the subsection of the rear lens group that is nearest the object side be the subsection that controls vibration of the image field.
However, in conventional lens construction, when it is attempted to provide a vibration-control function to one or more lens elements, the following problems may occur. First, in zoom lenses that divide the: master section into a front lens group and a rear lens group, when one attempts to provide a vibration-control function to one or more lens elements in the rear lens group, the mechanism for moving the lens elements becomes complex due to the rear lens group also being the one which provides the tracking adjustment function and the macro-imaging function.
In the second known technique discussed above, wherein the rear lens group is subdivided into a first lens subgroup that is moved along the optical axis during macro-imaging adjustment and a second lens subgroup that is moved along the optical axis during tracking adjustment, there is no known way to provide a vibration-control function to the rear lens group.
In the third known technique discussed above, wherein the rear lens group is subdivided into three lens subgroups, with the second (i.e., middle) lens subgroup not being involved with either the macro-imaging function or the tracking adjustment function, a lens element of this second lens subgroup van be moved normal to the optical axis in order to provide the vibration-control function. However, although there are fewer structural problems when a lens element of this second lens subgroup is provided with a vibration-control function, moving a lens element of this lens subgroup for vibration control is not ideal in terms of affect on optical performance since the light is not collimated at this position. Therefore, it is desired to develop a lens that can be provided with a vibration-control function without the occurrence of any structural or optical performance problems.
The present invention is an imaging lens that is suitable for use in a commercial grade television camera for business use, such as broadcasting. The object of the invention is to provide an imaging lens having a vibration-control mechanism in the master section, in addition to the imaging lens having a macro-imaging function and a tracking adjustment function in the master section.